The invention described herein relates to a method for photochemically reducing uranyl ion in solution in tri-n-butyl phosphate, and more particularly to an improvement in the Purex process for treating irradiated nuclear fuels to recover uranium and plutonium values therein wherein the U(IV) produced by the photochemical reaction of uranyl nitrate with tri-n-butyl phosphate reduces the plutonium partitioned in the tri-n-butyl phosphate from Pu(IV) to Pu(III).
The Purex process for the treatment of irradiated, spent nuclear fuels to recover uranium and plutonium values therefrom has been in large scale use for over 20 years and is currently employed, with minor variations, by most of the nuclear fuel reprocessing plants now operating throughout the world. It is based on solvent extraction, i.e., the partitioning of a solute between two immiscible liquids. Tri-n-butyl phosphate (TBP) extracts uranium in the +6 oxidation state as the uranyl (UO.sub.2.sup.2+) ion and plutonium in the +4 oxidation state from aqueous solution, while extracting the other components of spent nuclear fuel to a much lesser degree. Plutonium in the +3 oxidation state is more soluble in the aqueous phase so that the plutonium in the TBP may be selectively partitioned into an aqueous stream by reducing the Pu(IV) to Pu(III). In practice, approximately a 30% solution of TBP in a suitable diluent, e.g., kerosene or normal dodecane, is used as the organic phase. By the addition of appropriate oxidizers, reducers, and salting-out reagents, the uranium and plutonium can be removed from the dissolved fuel elements and separated from each other.
The prior art teaches that a suitable agent for reducing the plutonium from the +4 to the +3 oxidation state is ferrous sulfamate, Fe(SO.sub.3 NH.sub.2).sub.2, which is effective because Fe(II) rapidly reduces Pu(IV) to Pu(III) and because the sulfamate ion stabilizes Pu(III). But ferrous sulfamate has the disadvantage of introducing nonvolatile constituents, i.e., sulfur and iron, into the process wastes. These constituents increase the volume of radioactive wastes to be stored and may accelerate the corrosion of evaporators.
An alternative approach is the use of U(IV) as the reductant for the Pu(IV). It is known that the ferrous sulfamate can readily be replaced with uranium(IV) nitrate or uranium sulfate. See, e.g., Schlea et al., "Uranium(IV) Nitrate as a Reducing Agent for Plutonium(IV) in the Purex Process," E. I. du Pont de Nemours & Company Savannah River Laboratory report DP-808 (1963). The addition of these salts, however, also increases the volume of the waste and has the added disadvantage of altering the isotopic ratio of the uranium that is recovered.
To avoid these problems, electrolytic reduction of the Pu(IV) is used in a rather recent variation of the Purex process. This approach, however, requires the use of a special partitioning column containing the necessary electrodes. It will be readily apparent that in the event of a malfunction, repair of the column is not easily achieved because of the contamination produced by the uranium and plutonium.
A simpler approach is to use photolysis for the reduction of uranyl ion by organic reductants. See, e.g., J. C. Carroll et al., "The Photochemical Reduction of Uranium(VI) to Uranium(IV) Nitrate," Hanford Atomic Products Operation report HW-70543(1961). While such reduction is known to occur in the presence of a variety of organic reagents, the literature appears contradictory as to whether it has in fact been shown to occur with TBP used as the reductant. Thus, Kertes et al. expressly state that in nitric acid systems with either the neutral or acidic butyl esters of phosphoric acid, no photosensitized reduction of U(VI) takes place. See J. Inorg. Nucl. Chem., vol. 19, pp 359-362 (1961). But Minc et al. infer that in the presence of ultraviolet radiation, TBP reduces U(VI) in uranyl nitrate to U(V) and that this in turn undergoes a disproportionation reaction resulting in the formation of U(IV) and U(VI). See Nukleonika, vol. 5, pp. 33-45 (1960). Brzeski teaches that in the presence of strong ultraviolet radiation and the absence of oxygen, uranyl nitrate in solution in TBP is reduced to U(IV). He further indicates that such reduction does not occur with light in the visible spectrum. See Nukleonika, vol. 6, pp. 649-658 (1961).